Microelectromechanical systems (MEMS) are the integration of mechanical elements and electronics on the same chip using microfabrication technology similar to the IC process to realize high performance and low cost functional devices such as micro sensors and micro actuators.
MEMS is becoming an enabling technology in many fields as it enables the construction of devices or systems characterized by high performance, small size, small weight and low cost. Typical MEMS applications include: inertial measurement units such as micro accelerometers and micro gyroscopes; optical MEMS such as digital light processing (DLP) systems, micro optical switches and micromirrors for adaptive optics; and, RF MEMS devices such as micro RF switches, micro oscillators and micro varactors.
Micro actuators are important building blocks in constructing MEMS devices. There are four main actuation techniques used in MEMS, i.e., electrostatic, thermal, magnetic and piezoelectric. Among them, electrostatic actuation is the most used one because of its outstanding advantages such as low power consumption, simple structure, quick response, and especially high compatibility with IC fabrication technology. Micro electrostatic actuators can be categorized into two types, i.e., lateral (in-plane) actuators which move in the plane parallel to the substrate, and out-of-plane actuators which move in the plane perpendicular to the substrate. For lateral actuation or in-plane movement, combdrive types are preferred. The parallel-plate configuration is most suitable for vertical actuation or out-of-plane movement. Out-of-plane actuators are the subject of the present invention.
A conventional out-of-plane electrostatic actuator uses attractive electrostatic force and consists of two parallel plate electrodes: a fixed electrode and a moving electrode. The moving electrode is pulled down toward the fixed electrode by an attractive electrostatic force when a potential is applied between the two electrodes and it moves back to its original position due to a restoring force from supporting flexures when the voltage is removed.
The application of conventional parallel plate attractive electrostatic actuators is limited by the “pull-in” effect: when the displacement of the moving electrode exceeds ⅓ of the initial gap distance, the linear restoring force from the flexures cannot counteract the rapidly increasing nonlinear electrostatic attractive force between the fixed and moving electrodes, and as such the moving electrode sticks to the fixed electrode. A detailed explanation of the “pull-in” effect in conventional parallel-plate micro electrostatic actuators can be found in U.S. Pat. No. 5,753,911. Because of the “pull-in” effect the stroke of a conventional parallel-plate actuator is limited to less than one third of the initial gap distance between the fixed and moving electrodes.
Parallel-plate attractive micro electrostatic actuators are mainly fabricated by surface micromachining technology. In this technology, the initial gap distance between the fixed and moving electrodes is formed by a sacrificial layer such as silicon oxide, which is normally limited to a thickness of less than 2-3 micrometers. Therefore, the stroke of a conventional parallel-plate micro electrostatic actuator is limited to less than one micrometer (⅓ the thickness of the sacrificial layer).
In a number of MEMS applications, a stroke of the actuator as large as several micrometers is required. A significant effort has thus been dedicated to increase the stroke of the conventional parallel-plate electrostatic actuators. U.S. Patent Application Publication Nos. 2003/0103717 and 2004/0160118 present a method of using an elevation mechanism to raise the moving electrode to obtain a large initial gap between the fixed and moving electrodes, and therefore a larger stroke. By using the elevation mechanism a larger stroke is achieved at the price of increased fabrication complexity, lower space usage efficiency, lower production yield and higher driving voltage.
Another method to increase the stroke of conventional parallel plate attractive electrostatic actuators was disclosed in U.S. Patent Application Publication Nos. 2003/0011955 and U.S.2003/0117152. The method is based on using a special control circuit to realize a linear relation between the driving voltage and the gap distance, and therefore allows a large displacement. The largest stroke achieved by using this special control circuit is the full initial gap distance, which is normally limited in the range of 2-3 micrometers when standard surface micromachining is used to fabricate the parallel plate electrostatic actuator. Moreover, the special control circuit increases the cost of the chip.
Other methods were also developed to increase the stroke of the conventional parallel plate micro electrostatic actuators such as using second-order flexures (e.g., D. M. Burns and V. M. Bright, “Nonlinear flexures for stable deflection of an electrostatically actuated micromirror,” Proc. SPIE Conf. Vol. 3226, 1997) and a dual-gap structure (e.g., J. Zou et al., “Development of a wide tuning range MEMS tunable capacitor for wireless communication systems,” International Electron Devices Meeting, 2000).
All the attempts mentioned above to increase the stroke of the conventional parallel-plate attractive electrostatic actuators, are either not adequate to achieve a stroke large enough for many applications or are not compatible with standard surface micromachining technology and thus are difficult to implement in a batch-production process.
U.S. Pat. No. 5,541,465 discloses a design of special electrode arrangements for constructing cantilever actuators. This design has a serious drawback which limits it from being widely used as a bi-directional large stroke electrostatic actuator in MEMS devices, namely, the moving electrodes require electric potentials with opposite polarities thereby a movable insulation layer, such as silicon nitride or silicon oxide, has to be added to physically constrain and electrically insulate all the moving electrodes. These requirements complicate the fabrication process, lead to a deterioration of the performance of the component, and add wiring complexity. Moreover, no moving insulation layer is available in many commercial MEMS fabrication processes such as MUMPS (Multi User MEMS Processes), SUMMIT (Sandia Ultra planar Multilevel MEMS Technology), and so on.
An electrostatic actuator utilizing both attractive and repulsive forces can provide bi-directional movement of the electrodes. The total stroke of such a bi-directional electrostatic actuator includes two parts, i.e., the displacement of the moving electrode in the direction toward the fixed electrode and that in the direction away from the fixed electrode. Therefore the stroke is not limited by the initial gap distance. Hence, a large stroke can be achieved by the bi-directional electrostatic actuator. Bi-directional electrostatic actuator can separate sticking surfaces, thereby is able to make reprogrammable MEMS nonvolatile memory.
A bi-directional electrostatic actuator of the comb-drive type has been disclosed and is the subject of U.S. Pat. No. 6,771,001. As is common with the comb-drive configuration, the disclosed device provides large in-plane motion while minimizing out-of-plane motion.
A need, therefore, exists for an improved electrostatic actuator. Consequently, it is an object of the present invention to obviate or mitigate at least some of the above mentioned disadvantages.